This invention relates to a magneto-optic memory medium with which writing, reading and erasing of information are performed through application of a laser beam.
In recent years, a substantial amount of effort has been directed toward the development of an optical memory medium which satisfies various requirements including high density, large capacity and high speed access.
Of a wide range of various optical memory media, magneto-optic memory elements comprising a perpendicular magnetization layer as a memory material are most attractive thanks to its uniquie advantage that information can be erased after use and new information can be written thereon. The magneto-optic memory elements however have the disadvantage that they provide weak reproduced signals and exhibit bad signal-to-noise ratio (S/N ratio). Especially when reproduction of information relying upon reflected light from the magneto-optic elements, that is, the Kerr effect reproduction system is in use, the Kerr rotation angle of magnetic material is small and an improvement in S/N ratio is almost impossible. For this reason the prior art approaches were made in an attempt to increase the Kerr rotation angle; an improvement in magnetic material as a recording medium; and deposition of such a dielectric film as SiO and SiO.sub.2. It was reported in "J. Appl. Phys." Vol. 45, No. 8, August 1974, for example, that the Kerr rotation angle increased from 0.7.degree. to 3.6.degree. by deposition of a SiO coating on a MnBi magnetic thin film as an example of the latter. However, the deposition of such a dielectric coating on a magnetic film increases the Kerr rotation angle, but on the other hand decreases the amount of the reflected light and S/N ratio improved no more than about twice. An improvement in S/N ratio is, therefore, not expected to a theoretical extent. In the case that the dielectric thin film typically of SiO and SiO.sub.2 is deposited, it can not protect the magnetic material against corrosion. It is also impossible to sense recording bits when the diameter of the recording bits is of the order of 1 um and dust or other foreign objects of a diameter of approximately 1 um. It is therefore desirable that the thickness of the memory elements be between 0.5 and 2 mm for practical application. However, this requirement is in conflict with the above discussed requirement of increasing the Kerr rotation angle.
Meanwhile, an approach has been suggested; an amorphous magnetic film such as DyFe is deposited on a garnet substrate and information recorded on the DyFe film is transferred to the garnet substrate of good S/N ratio for readout (e.g., "Digest of the Fourth Annual Conference on Magnetics in Japan", 5a B-4). However, this method does not appear to be suitable for large capacity memory application because of difficulty in making a large area memory medium.
Apart from the foregoing problems, high density recording is indispensable for optical memory elements. Accordingly, since the diameter of the recording bits is of the order of 1 um as stated previously, survo technique including a focus survomechanism and a track survomechanism is necessary during writing, reading nd erasing. Otherwise, there is the need for a recording mechanism which is too complex and precise for practical application. Unlike the Philips video disk equipment of the MCA type which requires only reproducing information previously recorded, the magneto-optic recording device should write new information at the position where no information is contained, when the track survomechanism is in use. It is therefore desirable that guide tracks be formed in parallelism with signal-recording tracks for helping the survomechanism.